Communication system



4 Shets-Sheet 1 R. H. CAMPBELL COMMUNICATION SYSTEM Filed sept. 7, 1942pril 18, 1944.

April 18, 1944. R. H. CAMPBELL COMMUNICATION SYSTEM Filed Sept. 7, 19424 Sheets-Sheet 2 MHK (flipar Q @ZZ g4 i fj/.Ld-l/'rl April 18, 1944. R.H. CAMPBELL COMMUNICATION SYSTEM Filed Sept. 7, 1942 4 Sheets-Sheet 5April 1s, 1944.

R. H. CAMPBELL COMMUNICATION SYSTEM Filed Sept. '7, 1942 4 Sheets-Sheet4 umm,

Patented pr. 18, 1944 2,347,079 COMMUNICATION SYSTEM Richard H.Campbell, Racine, Webster Electric Company, poration o! DelawareApplication September 7, 1942, Serial No. 457,549

65 Claims.

The present invention relates to intercommunicating systems andparticularly to the provision oi' new and improved circuit arrangementsenabling an operator at one station to indicate to an operator atanother station that he is desirous of interrupting or breaking in" onthe operator at another station when the latter is talking to theformer. The invention also relates to the provision oi new and improvedcircuit arrangements for enabling an operator at one station to signalan operator at another that it is desired to communicate with him.

In intercommunicating systems and particularly intercommunicatingsystems of the loudspeaker type, there has-been developed a type ofsystem wherein a talk-listen s witch is utilized to condition a stationeither for transmission or reception. In some systems, alternateoperation of switches at two stations between their talk and listenpositions is required in order for communication to be carried onbetween the two stations. In other systems, the direction ofcommunication is controlled by the operation of a switch at one stationbetween its two positions. In systems of these types, it may happen thatthe operator at a station conditioned for reception desires to interrupttransmission by the station conditioned for transmission. Heretofore,this has not been possible and it is, therefore, one of the primaryobjects of the present invention to provide an arrangement whereby astation conditioned for reception may signal a station conditioned fortransmission that it is desired to interrupt or break in on the latter.

Another of the primary objects of the present invention is to providecommunication systems with signaling arrangements whereby onestation cansignal another that it is desired to communicate therewith and whereinthe called station is given a first distinctive signal that anotherstation desires to communicate therewith in the event the called stationis not busy and given another and diierent distinctive signal in theevent the called station is busy.

A further object of the present invention is to provide communicationsystems with signaling arrangements whereby the calling station is givendistinctive signals of the character aforesaid as to whether or not thecalled station is busy or not busy.

A further object of the present inventionis to provide a communicatingsystem with signaling arrangements of the character aforesaid in whichthe distinctive signals given in the called and calling stations inresponse to the busy or not busy condition of a called station may bereversed.

A further object of the present invention is to provide improved signalsupply voltage arrangements..

Wis., assignor to Racine, Wis.. a coricl. 17a-37) Another object of thepresent invention is to provide an improved ampliier for use withintercommunicating systems of theloudspeaker type. Other objects andadvantages of the present invention will become apparent from theensuing description in the course of which reference is had to theaccompanying drawings in which:

Fig. 1 is a schematic representation ofone station of anintercommunicating system provided with apparatus constructed andarranged in accordance with the present invention;

Fig. 2 is a similar representation o1' a further portion of the system,this portion including two additional stations provided with the sameequipment as station illustrated in Fig. 1;

Fig. 3 is a schematic representation of the power pack for supplyingpower to the ampliiier located at station I; l

Fig. 4 is a schematic representation of a voltage supply arrangement forsupplying the signaling voltages utilized to provide distinctive signalsat the called and calling stations dependent upon whether or not thecalled station is busy or not busy;

Fig. 5 is a schematic representation of a modied form of circuitarrangement for enabling one station to signal another that it isdesired to break in on transmission from the other station; and

Fig. 6 is a view similar to Fig. 5 of a further modiiication.

Referring now to the drawings and more particularly to Figs. 1, 2, 3,and 4, it may be well perhaps to mention that these four figuresillustrate a complete three-station communication system embodying thepresent invention. Space requirements prevent disclosure of the systemon a single sheet of drawings, but the system as a whole may beconveniently noted if Fig. 2 is placed to the right oi.Fig. 1 and Figs.3 and 4 below Fig. 1.

The system has been illustrated as including three stations althoughadditional stations could be readily added to the system in a mannerthat will become apparent after the system has been described in detail.The three stations of the system' have been indicated by referencecharacters I, 2, and 3. Each of the stations is provided withtranslating devices capable of being utilized both as microphones andloudspeakers such as the loudspeakers II and I2 and I3 which may be ofthe well known permanent magnet type. The stations are also providedwith amplifiers Il, I5, and I6, communication direction control switchesI1, I8, and I9, and station selector switches indicated generally byreference characters 2|, 22, and 23, all respectively.

The amplifiers are all of identical construction and therefore only theamplifier I4 at `station I has been illustrated in detail. While theamplifiers may be of any well known type, the ampliers illustrated areof the negative feedback type and are constructed and arranged inaccordance with one of the features of this invention to give an outputthat is considerably above the output obtained with conventionalamplifiers.

The amplifier I4 includes input transformer 24 to the primary winding ofwhich are connected the amplifier input conductors 25. The secondarywinding of transformer 24 is connected to the tubes 26 and 21 (of the6SJ7 type) which are arranged in parallel so that in the event one ofthe tubes fails for any reason whatever, the

amplifier will remain operative. The output of the tubes 26 and 21 issupplied to tubes 23 and 29 (of the 6J5 type) which 'are also connectedin parallel. The output of tubes 28 and 25 is supplied to the primarywinding of an interstage coupling transformer 30, the secondary of whichcomprises two windings 3I and 32 connected in push-pull arrangement withthe power tubes 33 and 34 (of the 6V6GT type). The anodes of the twotubes are connected to the primary winding 35 of an output transformer36 and the midpoint of the primary winding is connected by a conductor31 to a conductor 38 through which the supply of B voltage is led to theamplifier. It may be well to note at this point that the heaters of thevarious amplifier tubes are continuously supplied with power but that Bor plate voltage is not supplied to them except when the communicationcontrol switch I1 is in its lowermost or talk position as will bedescribed in greater detail hereinafter.

The output transformer 36 includes a rst secondary winding 4I to theterminals of which are connected a pair of output conductors 42. Thetransformer also includes a second secondary winding 43, the midpoint ofwhich is grounded to the chassis of the amplifier by a conductor 44 andthe opposite terminals of which are connected by conductors 45 and 46 toadjacent terminals of windings 3l and 32 of the interstate couplingtransformer 30. The other terminals of the latter are connected byconductors 41 and 43 to the grids of tubes 33 and 34, respectively.

- The grids of the two tubes are connected through a condenser 49. thepurpose of which will now be described.A

The output transformer secondary winding d3 is utilized as a source ofnegative feedback for the push-pull connected power output stage of theamplifier. As frequencies of about 15,000 cycles may be shifted in phaseenough to cause a positive instead of negative feedback and thus producean undesirable oscillation, the condenser 49 is used to bypass or reducethese higher frequencies enough to prevent excessive positive feedbackwhich might otherwise produce oscillation. When the condenser is usedlthe amplifier can be adjusted to operate as a Class AB-2 amplifier. Theamplifier thus draws grid current at high volume, that is, when theaudio signal voltage applied to the grids of tubes 33 and 34 exceeds thenormal grid bias voltage. The grid return circuit including conductors45 and 45 is of low resistance and, consequently, the power outputobtainable from the amplifier output stage is considerably increased. Afurther advantage of 'the amplifier circuit arrangement is to balancethe grid current iiowing through the secondary winding 43 so that itdoes not deleteriously affect the nature of the amplifier output.

The amplifier also includes a volume indicator in the nature of a neontube I connected across the primary winding 35 in series with the high-value (50,000 to 100,000 ohms) resistors 52 and 53, which preventdirect shorting lof the ampliner B supply in the event the tube orsocket should ground to the chassis. Should such grounding take place,one of the resistors would probably be destroyed but the powertransformer and rectifier tube saved.

The amplifier is supplied with power through a power pack indicated`generally by reference character 54 and illustrated in Fig. 3. The powerpack includes a transformer 55, the primary winding 5B of which isconnected to a suitable source of alternating current through conductors51. A pilot light in the form of a neon tube 55 is connected acrossconductors 41 through the resistors 59 and 60. The transformer isprovided with three secondary windings, the first 5I of which isutilized as a source ofpower for the cathode heaters of the ampliertubes. A secondary winding 62 is used as a source of filament heatingcurrent for the rectifier tubes 63 and 64, and a third winding 65 isutilized as a source of anode voltage for the rectifier tubes, which, itmay be noted, are connected in conventional manner for full waverectification. The B supply line 38 is connected to the filaments of thetubes 63 and 54 in customary manner. Each of the amplifiers I5 and I6has a similar power pack associated with it and indicated in each caseby the reference character 54.

The communication control switches I1, I5, and I9 are operable' betweena listen position, in which they are indicated, to a talk position whichis a lower position. The switches are preferably biased from their talkto their listen positions by springs 61. In the listen positions of theswitches, the loudspeakers at the respective stations are connected toan interstation conductor individual thereto and in the talk positionsthe loudspeakers are connected to the inputs of the amplifiers and theoutputs of the amplifiers are connected to the selector switches andthence through operated selector switches to the interstation circuitsindividual to other stations.

Referring now to Fig. 1 and station I, it may be noted that thecommunication control switch I1 includes a pair of switch blades 1Iconnected directly through the pair of conductors 12 to the voice coilof loudspeaker I i. In the indicated position of the switch I1, theloudspeaker is connected to an interstation circuit individual tovstation I andy including a pair of conductors 'I3 through the switchblades 1I, through an adjustable volume vcontrol 14 and a transformer15. The primary winding 16 of the transformer has two terminalsconnected tothe conductors 13. One terminal of the secondary winding 11of the transformer is connected by conductor 18 to a contact associatedwith the lowermost switch blade 1 i, while the movable contact 14 of thevolume cont'rol is connected by conductor 19 to a contact associatedwith the other of switch blades 1 I. Volume control is effected bymoving the contact 14 to vary the effective portion of the secondarywinding 11.

In the indicated position of switch I1, the amplier output conductors 42are open circuited by the pair of switch blades 8| to which they areconnected.

In the lowermost position of the communication control switch, theloudspeaker II is connected to the amplifier input through the switchblades 1I in their lowermost positions and the input conductors 25. Atthis time, the amplifier switch blades 8| (in their lowermost positions)to` an output circuit 82 leading to the station selector switches 2|through which the amplifier output may be supplied to interstationcircuits individual to the other stations of the system. In the systemherein illustrated, there are but two other s uch stations and thereforetwo station selector switches have been illustrated. These are theswitches 83 and 84 individual to stations 2 and 3, respectively.

When it is desired to communicate with station 2, station selectorswitch 83 is operated from its indicated lowermost position to adisplaced position wherein the output circuit 82 is connected to aninterstation circuit 85 individual to station 2, this connection beingmade through switch blades 86. When it is desired to communicate withstation 3, the station selector switch 84 is operated so that the switchblades 81 thereof connect the output circuit 82 to the interstationcircuit 88 individual to station 3. The station selector switches havealso associated therewith additional circuits controlled thereby forsignaling called stations, and also station I, in a distinctive mannerdependent upon whether or not the called station is busy or not busy, aswill be described in detail hereinafter.

The communication control switch I1 includes an additional switch blade89 operable into a circuit closing position to supply B voltage to theamplifier. The switch blade 89 breaks the B supply line 38 when theswitch I1 is in its uppermost or listen position and completes thecircuit when it is in its lowermost or talk position.

Referring now to Fig. 2 and station 2, it may be noted that thecommunication control switch |8 includes a pair of switch blades 9|connected through conductors 92 to the voice coil of loudspeaker |2. Inthe indicated position of switch I8, the loudspeaker is connected tointerstation circuit 85 individual to station 2 through a volume control93 and transformer 94. iAt this time, the switch blades 95 of switch I1open circuit the amplifier output to which they are connected throughconductors 96. In the lowermost position of the communication controlswitch, loudspeaker I2 is connected to the amplifier input throughswitch blades 9| and the conductors 91. The amplifier output conductors96 are at the same time connected to an output circuit 98 leading to astation selector switch 99 individual to station I and a stationselector switch individual to station 3. When the station selectorswitch 99 is closed, the output circuit 98 is connected through switchblades I0| to the interstation circuit 'I3 individual to station andwhen station selector switch |00 is closed, the output circuit isconnected through switch blades |02 to interstation circuit 88individual to station 3.

The communication control switch includes also a switch blade |03adapted when switch I8 is iny In the lowermost position of thecommunication control switch, loudspeaker I3 is connected to theamplifier input through switch blades |03 and input conductors ||0. Theamplifier output conductors |09 are at the same timeconnected to anoutput circuit |I| leading to a station selector switch |I2 individualto station I and a station selector switch ||3 individual to station 2.When station selector switch I I2 is closed, the output circuit I I0 isconnected through switch blades I I4 to interstation circuit 13individual to station I and when switch ||3 is closed, circuit isconnected through switch blades II5 to interstation circuit 85individual to station 2. The switch I8 also includes a switch blade ||5adapted to complete the B supply connection to the amplifier in the talkposition of the switch.

Communication between any two stations of the system is carried on byalternate operation of the communication control switches at the twostations between their indicated and lowermost positions, i. e., betweentheir listen and talk positions, provided of course that the stationselector switches at the two stations have been closed to complete theinterstation circuits between them. Suppose. for instance, that stationI is communicating with station 2 (station selector switches 83 and 99having been closed), and that the operator at station I is talking tothe operator at station 2, then if the operator at station 2 desires tobreak-in on the operator at station I, he can` not do so even though heoperates the communication control switch I8 thereat to its talkposition. The reason for this is that under these conditions theloudspeakers at both stations would be connected to the inputs of theamplifiers thereat and no loudspeakers would be connected to the outputsof the amplifiers.

One of the important features of the present invention is to provide asignaling arrangement whereby under the conditions mentioned above. anindication is given at a station conditioned for talking that thelistening station desires to interrupt or breakin. It is preferred thatthe signal be given as by the lighting of a lamp and consequently thevarious stations have been provided with signaling means consisting ofneon tubes that are adapted to be lighted to indicate the break-in.These neon tubes are indicated by the reference characters ||6, ||1, and||8 (of the type NE17 manufactured by the General Electric Company), atstations I', 2, and 3, respectively. The tube |I6 has one terminalconnected through a voltage limiting resistor ||9 and conductor |20 to asource of direct current voltage which may be the conductor |2| throughwhich vplate voltage is supplied to tubes 26 and 21 of the amplien Theother terminal of tube 6 is connected by a conductor |22 to the midpointof the primary winding 16 of transformer 15. The tube is shunted by aresistor |23 having a value of about 100,000 ohms and the purpose ofwhich is to decrease the voltage across the tube terminals so that itwill not be lit as a result of resistance leakage and capacity betweenthe interstation line 13 and ground, to which the negative side of the Bvoltage is connected.

In similar manner, one terminal of tube |I1 at station 2 is connectedthrough resistor |24 and conductor |25 to a source of direct currentvoltage while the other terminal is connected by conductor |26 to themidpoint of the primary winding'of transformer 94. Tube ||1 is bridgedby the resistor |21.

The tube ||8 at station 3 is likewise connected through resistor |28 andconductor |29 to a source of direct current voltageand by conductor |80to the midpoint of the primary winding of transformer'jli. The tubeIll'is also'bridged"` by the resistor I3|.`

I, the ground connection l|34 is made to the midpoint of a pair ofresistors |35 and |35 connected across the amplier output conductors 42.At station`2, the groundconnection |31 is made at the midpoint ofresistors |38 and |39 connected across-the amplifier output conductors98. At station 3, the ground connection |40 is made at the midpoint ofresistors |4| and |42 connected across the amplifier output conductorsThe arrangement of the neo n tube break-in signals is such that when thetalk-listen switches at two stations in communication with each otherare both in their listen positions, or that at one station is in itslisten position and that at the other in its talk position, the neontubes are not lit. However, when both the switches are in their talkpositions, thus indicating that the station conditioned for receptiondesires to interrupt or break in on the station conditioned fortransmission, then the tubes at both stations are lit.

When the talk-listen switches are in their listen or indicatedpositions, then the neon tubes are not supplied with voltage since noplate voltage is supplied-to the amplifiers as the plate supplycontrolling switch blades 89, |03, and |55 at the three stations are incircuit opening positions. When the talk-listen switch at one station isin its tall'. position (for instance, switch I1 at station I) and thetalk-listen switch at another station (for instance, switch I8 atstation 2) is in its listen position then, while the B supply voltage isconnected to tube I I5 at stationw-I, there is no complete energizingcircuit for this'tube because the switch blades 95 at station 2, throughwhich the energizirg circuit for the tube is completed to ground, are incircuit opening position.

When both of the switches |1 and |8 are in their talk positions, whichpositions they would occupy when the operator at station 2 desires tointerrupt the operator at station I, then the tubes ||5 and I I1 atstations and 2 are both lit. The energizing circuit for tube ||5 extendsfrom the B supply conductor 38 at station I, conductors |2I, |20 andresistor ||9 to one terminal of tube IIB. The other terminal of the tubeis connected to ground through conductor |22, primary winding oftransformer 15 at station l, interstation circuit 13, the stationselector switch blades IUI at station 2, switch blades 95, conductors96, resistors |38 and |39 and the ground connection |31 at station 2.The various-ground connections at the stations may be connected to asingle conductor or to shielding around a multiple conductor cable. Theenergizing circuit for tube ||1 at station 2 extends from the B supplyat that station through conductor |25 and resistor |24 to one terminalof the tube, and the other terminal of the tube is connected to groundthrough conductor |25, the midpoint of the primary winding oftransformer 94 at station 2, interstation circuit 85, switch blades 85,switch blades 8|, resistors |35 and |35,'and the ground connection |34at station The operator at station is thereupon advised by the lightingof tube ||5 that the operator at station 2 desires to break in.

In the event station 2 had been transmitting to station and the operatorat the latter desired to break in,v then both of the talk-listenswitches would again be operated to their talk positions, with theresults described above. namely, that both tubes ||5 and ||1 at the twostations-would be lit. j

Inasmuch as the apparatus at station 3 corresponds to that at stationsand 2, the desire to break in is indicated'in the same way if station 3is communicating with either one of the other stations.

One of the advantages of the described signaling arrangement is that itdoes not require any additional interstation conductors other than thoseutilized in the communication system. Furthermore, the connection of thetubes to the midpoints of the transformers and of the ground connectionsto the midpoints of resistors in the amplifier output circuits providesa balanced circuit arrangement to eliminate undesirable consequencesthat might otherwise result from the use of the communication circuitsfor signaling purposes.

Another of the important features of the present invention resides inthe provision of signaling arrangements whereby a signal of a firstdistinctive character is actuated at both the calling and calledstations upon the selection of a station that is not busy and a signalof another characteristic is actuated at both stations in the event theselected station is busy. In the instant embodiment, the rst of thesignalsof distinctive characteristic is a flashing signal light and thesecond is a, steady light.

Each of the stations is provided with a light source, preferably a neontube (preferably of the same type as the tubes |I8 to H8) for each ofthe stations in the system. Thus station is provided with tubes |45 and|45 corresponding to stations 2 and 3, respectively; station 2 isprovided with tubes `|41 and |48 corresponding to stations and 3,respectively, and station 3 is provided with tubes |49 and |50corresponding to stations and 2, respectively.

4Upon the selection of one station by another, the tubes correspondingto the two stations are supplied with either a periodic or steadyvoltage dependent upon whether or not the called station is not busy orbusy. These two voltages are supplied from a centrally located signalvoltage supply source |5I (see Fig. 4) which will be described in detailhereinafter. From an inspection of this Fig. 4, it will be noted thatthree conductors |52, |53, and |54 lead from the supply source |5I toall stations of the system. The conductor |52 is a common conductor,conductor |53 is a conductor supplying the periodic voltage, andconductor |54 is a conductorl supplying the steady signal voltage.

The station condition responsive signaling arrangement is controlled inresponse to the positions of the station selector switches at thevarious stations. Referring now first to station I, it may be noted thatthe station 2 selector switch 83 thereat includes switch blades |55 and|55 operable simultaneously with the switch blades 88. The neon tube |45is connected tc switch blade |55 through a. resistor |51 and conductor|58. The other terminal of the tube is connected by a conductor |59 tothe corresponding terminal of the tube |41 at station 2.

In the unactuated position of selector switch 83, switch blade |55completes a circuit to a branch conductor |5| connected to the commonconductor |52 and switch blade |55 interconnects conductors. |62 and|63. The latter is a branch conductor leading to the conductor |53through which the periodic voltage is supplied to the system, and theformer leads to a contact associated with selector switch 84, as will bedescribed shortly.

In the actuated position of selector switch 83, the signal circuitcontrolling switch blades |55 and |58 are moved upwardly. In thisposition, switch blade |55 interconnects the aforesaid conductor |62 anda branch conductor |64 leadingto the source of steady signal voltage,that is, conductorv |54, and switch'blade |56 interconnects conductor|58 and a conductor |65 leading to both stations 2 and 3.

Station1 selector switch 84 at station includes switch blades |61 and|68 operable simultaneously with the switch blades` 81. The neon tube|46 is connected to switch blade |68 through a resistor |69 andconductor 1|. The other terminal of the tube is connected by a conductor|12 to the corresponding terminal of tube |49 at station 3. In theunactuated position of selector switch 84, switch blade |68 completes acircuit to branch conductor |6| and the common conductor |52, whileswitch blade |61 interconnects the lpreviously referred to conductor |62and a conductor |13 leading to both stations 2 and 3. In the actuatedposition of selector switch 84, the switch blade |81 interconnectsconductors |64 and |13 and switch blade |68 interconnects conductors |1|and a conductor |14 leading to both stations 2 and 3.

The signal circuit controlling arrangement at station 2 is similar tothat at station I. The station selector switch 99 includes a pair ofswitch blades |15 and |16, operable simultaneously with the switchblades |I. The neon tube |41 is connected to switch blade |16 throughresistor |11 and conductor |18. The other terminal of the tube isconnected to the previously referred to conductor |59. In the unactuatedposition ofselector switch 99, switch blade |16 completes the circuit toa branch conductor |19 leading to the common conductor |52 and switchblade |15 interconnects conductors |8| and |82. 'I'he latter is a branchconductor leading to the flashing voltage supplying conductor |53, andthe former leads to a contact associated with selector switch |00. Inthe actuated position of selector switch 99, the signal circuitcontrolling switch blades |15 and |16 are moved upwardly. In thisposition. switch blade |15 interconnects the conductor |8| and a branchconductor |83 leading to the steady voltage supplying conductor i 54,and switch blade |16 interconnects conductors |18 and |84, the latter ofwhich is connected to the conductor |13, which leads to both stationsand 3.

Station selector switch |00 includes the switch blades |85 and I 86,operable simultaneously with switch blades |02. through resistor |81 andconductor |88 to switch blade |86. The other terminal of the tube isconnected by a conductor |89 to the corresponding terminal of tube |50at station 3. In the unactuated position of selector switch |00, switchblade |86 completes a circuit from conductor |88 to the branch conductor|19 leading to the common conductor |52 and switch blade |854interconnects branch conductor |8| and a conductor I9I which isconnected tothe conductor |65 leading to the other stations of thesystem. In the actuated position of selector switch |00, switch blades|85 and |86 are moved upwardly. In this position,

The neon tube is connected blade interconnects conductors |9| and the 75branch conductor |83 connected to the steady voltage supply line |54 andblade |86 interconnects conductors |88 and a branch conductor |92connected to the conductor 14.

At station 3, the station selector switch ||2 includes switch blades and|96 operable simultaneously with switch blades ||4. The neon tube |49 isconnected to switch blade |96 through a resistor |91 and conductor |98.The other terminal of the tube is connected by 4conductor |22 to thecorresponding terminal of tube |46 at station In the unactuated positionof selector switch 2, switch blade |96 completes a circuit fromconductor |98 to a branch conductor |99 leading to the common conductor|52 and switch blade |95 interconnects conductors 20| and 202, thelatter of which is a branch conductor leading to the periodic voltagesupplying conductor |53, and the former of which leads to selectorswitch H3.. In the actuated position of selector switch I2, switchblades |95 and |96 are moved upwardly to a position wherein blade |95interconnects conductor 20| and the branch conductor 203 leading to thesteady Voltage supplying line |54 and blade |96 interconnects conductors|13 and 98.

The station selector switch ||3 at station 3 includes the switch blades205 and 206 operable simultaneously with switch blades I |5. The neon'tube |50 is connected to switch blade 206 through a resistor 201 andconductor 208 and its other terminal is connected to the conductor |89con nected to the corresponding terminal of tube |48 at station 2. Inthe unactuated position of selector switch H3, switch blade 206completes the circuit from conductor 208 to the branch conductor |99connected to the common conductor |52 and switch blade 205 interconnectsconductors |14 and 20|. In the actuated position of selector switch ||3,switch blades 205 andv 206 are moved upwardly to positions wherein theformer interconnects conductors |14 and 203 and the latter interconnectsconductors 208 and |65.

In order to prevent excessive voltage drop across the neon tubes whensignal voltage is applied thereto, due to the capacity between the wiresof the signaling arrangement, each of the tubes is bridged by a resistor2|0. These resistors may have a value of about 100,000 ohms, whereas theresistors |61, |69, etc., in series with the tubes, may have a value ofabout 10,000 ohms. The resistors 2|0 also enable radio frequencyvoltages, which may be induced in the wires because of high power radiosutilized on ships, to leak out. If this were not done, these inducedvoltages might be suicient to light the tubes and give erroneoussignals.

The signal voltage supply means |5| illustrated in Fig. 4, to whichreference is now had, includes a transformer 2 having a primary. winding2|2 4and three secondary windings 2|3, 2|4A and 2|4B. The primarywinding may be connected to a suitable source of alternating currentvoltage. Secondary winding 2I3 is connected to the plates of a pair ofrectifier tubes 2|5 and 2|6 arranged for full wave rectication and theIllaments of which are independently connected to the secondary windings2|4A and 2|4B. The output of tube 2|5 is supplied to the steady voltageconductor |54, across'which and the common conductor is connected acondenser 2|1. The output of the other tube 2|6 is supplied to aconductor 2|8, across which and the common conductor is connected acondenser 2|9, and the conductor 2|3 is periodically connected to theconductor |53.

'I'he periodic voltage is supplied to conductor |53 by a novelarrangement including a relay 220 having a condenser 22|i connected inshunt thereto. A pair oi resistors 222 and 223 are connected in serieswith the parallel connected relay and condenser. This series-parallelcircuit is in turn shunted by a condenser 224. The relay 220 operates atwo-position switch blade 225 adapted in its indicated position (towhich'it is suitably blasedl to connect conductor 2 I8 to the relaycil-,cuit through a resistor 226. When the relay winding is energizedsufllciently, the switch blade 225 is actuated to connect conductor 2|8yto the conductor |53 through which, it may be remembered, a periodicvoltage is supplied to the signal circuit.

A neon pilot light 221 is connected across conductors |53 and |52 inseries with resistors 228 and 229 to give an indication that the voltagesupply means is operating.

When the switch blade 225 occupies its indicated position, a steadydirect current voltage obtained from conductorl I8 is utilized to chargecondensers 22| and 224 and to energize relay 220. The condenser 224ischarged to a, higher voltage than condenser 22| by reason of theresistors 222 and 223 being located in circuit with condenser 22|. Asthe condensers are charged, an increasing voltage is supplied to therelay winding 220 until it becomes energized suieiently to move switchblade 225 to disconnect conductor |54 from the relay circuit and toconnect it t conductor |53. When this occurs, the relay winding remainsenergized and continues to pull switch blade 225 downwardly because ofthe discharge of condensers 22| and 224 through the winding. Condenser22| discharges rapidly, but condenser 224, which is charged to a highervoltage, discharges more slowly because of the series resistors 222 and223. Consequently, switch blade 225 is held in its lowermost positionuntil the bias applied thereto overcomes the diminishing force appliedto it by the decreased current ilowing through the winding.

The described arrangement is very advantageous in that the ratio of thetime voltage is supplied to conductor |53 to the time that no voltage issupplied to the conductor is relatively independent of the powerrequired to move switch blade 225 because the energization of the relayis increased to effect operation of the switch blade in the event thelatter tends to stick. If the blade tends to stick, then the condensers22| and 224 are charged for a slightly longer time and consequently theenergization of the relay is increased sothat more power is availablefor movement of the switch blade. the ratio is independent of the load(the number oi neon tubes energized) as condensers 22| and 224 areentirely disconnected from the load circuit When the switch blade-225 ismoved from its indicated position, and the discharge is through therelay winding and not through the load which may be variable.

in order to vary the ratio of the lengths of time that the switch bladeremains in its two operated positions, the circuit constants of theelements in circuit with the relay winding as well as the resistance ofthe relay winding itself may be varied. To increase the length of timethat the switch blade remains in its actuated position, the condensercapacities may be increased so of time. In addition. the resistances ofresistors 222 and 223 or oi the relay winding itself may be increased.

Excellent results have been obtained with an arrangement wherein theconductor 2|-8 has a voltage of about 200 volts, the relay winding 220has resistance of about 5,000 ohms, and resistors 222, 223, and 22| haveresistances of about 5,000 ohms, and the condensers 22| and 224 acapacity oi about 4 microfarads. When thus arranged, the neon tubes,when supplied with the periodic voltage, iiashabout 4 times per secondand the ratio oi' the period of illumination to the period of darknessis about 60 to 40.

If it is .desired to reverse the character of the distinctive signalsgiven at the various stations upon the selection of a busy or a not busystation; a reversing switch 230 (see Fig. 1) may be provided whereby theconnection of conductors' conductor |54 is periodically supplied withvolt-l age.

Another advantage of the arrangement is that the steady voltage appliedto conductor |54 from rectifier tube 2|5 is substantially independent ofthe load connected across the periodic voltage supply conductor |53.Thus, a change in load in either circuit is reflected into the othercircuit only by a slight voltage drop across the transformer itself.This would not :be the case if the steady voltage was taken from thesame rectiiier as the periodic voltage.

In operation oi' the system, it is contemplated that the amplifiers ofthe various stations will be conditioned for immediate operation as bythe connection of the power pack transformers 55 to suitable source ofalternating current. When thus connected, the operators at the variousstations will be apprised of the operative condition oi the ampliiiersby the glowing of the neon tubes 5B. The signal voltage supplying means|5| also has its transformer 2li connected to a suitable source ofalternating current so that the steady and periodic signaling voltageswill fbe at all times supplied to the conductors |54 and |53. The neontube 221 which is connected to the periodic voltage supplying conductor|53 thus ashes periodically to indicate that the voltage supplying meansis operating.

Furthermore,

Under normal conditions, i. e., when none of the stations in the systemis in us'e, the communi cation control switches Il, i8, and i9 are intheiil uppermost indicated positions so that the loud?- speakers il, i2,and I3 are connected through their associated volume controls andtransformers to the interstation circuits 13, 35, and 08 individu; ai tothe three/stations. The amplifier output circuits are opened by thecommunication control switches and no plate voltage is supplied to theampliers although the heaters of the various amplier tubes are suppliedwith current to make the ampliers ready for instantaneous operation.

The break-in signaling tubes H6, H1, and ||8 at the stations aredeenergized. The various station selector switches are also in theirunactuated positions and the signaling tubes |45 to |50 are deenergized,the pairs of tubes |45 and |4'|l |46 and |43, and |48 and |50, eachbeing connected in series to the common conductor |52. The tubes thatthe condensers discharge for a longer period |45 and |41 are connectedin series to the comcalling station and the called station 2.

mon conductor through a circuit including the branch conductor |6| atstation I, switch blade |51, conductor |58, resistor |51, the tube |45,interstation conductor |59, the tube |41 at station 2, resistor I 11,conductor |18, switch blade |16, and the .branch conductor |19 atstation 2 which is also connected to conductor |52. Tubes |46 and |49are similarly connected to the common conductor through a circuitincluding the branch conductor |6| at station I, switch blade |68,conductor |1I, resistor |69, the tube |46, interstation conductor |12,the tube |49 at station 3, resistor |91, conductor I 98, switch blade|96, and branch conductor |99 at station 3. Tubes |48 and |50 areconnected to the common conductor through the circuit including branchconductor |19 at station 2 switch blade |86, conductor |88, resistor|81, and the tube |48 at station 2, interstation conductor |89, the tube|50 at station 3, resistor 201, conductor 208, switch blade 206, and thebranch conductor |99.

Assuming rst that the operator at station I desires to communicate withthe operator at station 2 and that the latter station is not busy, thenthe operator at station I actuates selector switch 83 and a flashingsignal is produced at both tige T e operator at, station I is thereuponadvised that station 2 is not busy and the operator at the latterstation is apprised by the hashing signal light that another stationdesires to communicate therewith. A flashing signal is supplied at both'stations by the connection of the tubes |45 and |41 at stations I and 2,respectively, to the conductor |53 which, it may be remembered, issupplied with a periodic or intermittent voltage. The energizing circuitfor the two tubes is as follows: conductor |52 at station 2, branchconductor |19, switch blade |16 (which is in its indicated position asselector switch 99 has not yet been actuated) conductor |18, resistor|11, tube |41, interstation conductor |59, tube |45 at station I,resistor |51, conductorv |58, switch blade |56 in its uppermostposition, interstation conductor |65, conductor |9| at station 2, switchblade |85 in its unactuated position, conductor IBI, switch blade |15 inits unactuated position, and branch conductor |83 which is connected toconductor |53 ion 2. atl'isittmthis time, the operator at station 2isapprised of a call by the ashing of a signal light and if the operatorat station I desires, .he can also call the operator at station 2 byusing. the intercommunicating apparatus. In doing this, it is necessaryfor the operator at station I simply to ,operate the communicationcontrol switch I1 thereat to its lowermost or talk position. When thisis done, the loudspeaker IIA at station I is connected to the input ofthe ampliner I4 and the output of the amplier is connected toloudspeaker |2 at station 2 through the switch blades 8| of switch I1 atstation I, the actuated station selector switch blades 86 and theinterstation circuit 85 individual to station 2 and to which loudspeakerI2 is normally connected through its associated volume control 93 andtransformer 94. Thus if the operator at station 2 hears the call, he canrespond thereto, but if he is not where he can hear the call, thestation selector switch at station I can be left in its actuatedposition so that the signal light at station I will flash periodicallyuntil the operator at station 2 responds.

In order for the operator at station 2 to reply former rst to actuateselector switch 99. When the station selector switches 83 and 99 atstations and 2 have both been actuated, the tubes |45 and |41 are bothdeenergized. The tubes are connected in series to the steady voltagesupplying conductor |54, but there is no circuit to the commonconductor, and the tubes are deenergized. 'I'he circuit connections ofthe two tubes at this time are as follows: conductor |54 at station 2,conductor |83, switch blade |15 in its uppermost position, conductorI8I, switch blade |85 in its unactuated position, conductor |9|,interstation conductor |65, switch blade |56 at station 2 in itsuppermost position, conductor |58, resistor |51, tube |45, interstationconductor |59, tube |41 at station 2, resistor |11, conductor |18,switch blade |16 in its uppermost position, conductor |84, interstationconductor I 13,. switch blade |61 at station I in its unactuatedposition, conductor |62, switch blade |55 in its uppermost position,'and branch conductor |64 leading to conductor |54 at station I.

After the station selector switches at stations I and 2 have beenoperated as described above, then communication between the two stationsmay be carried on by the alternate operation of the communicationcontrol switches I1 and I8 between their talk and listen positions. IntheY event one of the stations is conditioned for transmitting to theother by the operation of its switch to its talk position and theoperator at the other station desires to break in on the one, then allthat is necessary is for the operator a't the other station to actuatethe switch thereat to its talk position. Both the switches are thereuponoperated into their talk positions with the result that the break-insignal tubes II6 and II1 at the two stations are energized in a mannerheretofore t0 a call from station I, it ls necessary for the 75described in detail.

It will now be assumed that stations I and 2 are in communication witheach other as described above and that station 3 selects one of the twostations, for instance, station 2. To select station 2, the operator atstation 3 actuates station selector switch I`I3, whereupon tubes |48 andI 50 at stations 2 and 3 are supplied with a steady voltage and thusglow steadily to indicate to the operator at station 3 that station 2 isbusy and to indicate to the operator at station 2 that station 3 desiresto communicate therewith. At this time, tubes |48 and |50 are connectedin series acrossconductors |52 and |54, the circuit being as follows:conductor |52 at station 2, branch conductor |19, switch blade |86 inits unactuated position, conductor |88, resistor |81, tube |48,interstation conductor |89, tube |50 at station 3, resistor 201,conductor 208, switch blade 206 in its actuated position, interstationconductor |65, conductor |9| at station 2, switch blade |85 in itsunactuated position, conductor I8I, switch blade |15 in its uppermostposition, and conductor |83 which is connected to the -steady voltagesupply-l ing conductor |54.

At this time, the operator at station 2 has the option of continuing tocommunicate with station I or to communicate with station 3. Shouldtheoperator at station 2 decide to communicate with station 3 for anyreason whatever, as such, for instance, as a belief that the call fromstation 3 is of an urgent or emergency character, then the operator atstation 2 can return to its unactuated position the number I stationselector switch 83, and operate the station 3 selector switch 84. Ifthis is done, the tubes |48 and |50 at stations 2 and 3 areextinguished, whereas tubes |45 and |41 at stations I and 2 will besupplied with a steady voltage and glow continuously. 'Ihus theloperator at station I is advised of the changeover in the status ofstation 2', namely, that this station has selected some other stationfor communication. The tubes |48 and |50 at stations 2 and 3 areconnected to the: steady voltage supplying conductor |54 but not to thecommon conductor and are consequently deenergized. This circuitconnection is as follows: conductor. |54 at station 2, conductor |83,switch blade |85 in its uppermost position, conductor |8I, interstationconductor |85, switch blade 205 at station 3 in its uppermost position,conductor 208, resistor 201, tube |50, interstation conductor |88, tube|48 at station 2, resistor |81, conductor |88, switch blade |86 in itsuppermost position, conductor |92, interstation conductor |14. switchblade 205 at station 3 in its uppermost position, and conductor 203which is connected to'conductor |54.

Remembering now that station selector switch 88 at station 2 has beenreturned to its indicated position, then the circuit whereby tubes |45and |41 at stations I and 2 are supplied with a steady voltage uponstation 2 giving precedence to station 3 over station I, is as follows:conductor |52 at station 2, conductor |18, switch blade |15 in itslowermost position, conductor |18, resistor |11, tube |41, interstationconductor |59, tube |45 'at station I, resistor |51, conductor |58,switch blade |58 in its uppermost position, interstation conductor |85,conductor |8| at station 2, switch blade |85 in its uppermost position,and conductor |83 which is connected to conductor l54at station 2.

.In the event two stations of the system select a third at approximatelythe: sarne time, and prior to the third having responded to a call fromone of the two, then both neon tubes at the third station and the tubescorresponding to the third station at the two other stations are allsupplied with periodic voltage to give flashing signals. Tubes |45 and|41 at stations I and 2 are connected in series across the commonconductor |52 and the periodic Voltage supplying conductor |53 in themanner heretofore described in conjunction with the selection ofstation2 by station I. The tubes |58 and |50 at stations 2 and 3 areconnected across the common conductor and conductor |53 through acircuit that is as follows: conductor |52 at station 2, conductor |19,switch blade |88 in its unactuated position, conductor |88, resistor|81, tube |48, interstation conductor |88, tube |50 at station 3,resistor 201, conductor 208, switch blade 205 in its uppermost position,

' interstation conductor |65, conductor IBI at station 2, switch blade|85 in its unactuatedposition, conductor |8I, switch blade: |15 in itsunactuated position, and conductor |82 which is connected to conductor|53.

Under the above circumstances, the called station (station 2) has theoption of communicating with either one of stations l or 2. Assuming,rst, that station 2 selects station by the operation of selector switch99, then tubes |45 and |41 at stations I and 2 are extinguished andtubes |48 and |50 are connected across the common conductor and thesteady voltage supplying conductor |54 so that the latter tubes glowcontinuously. The circuit connections existing at this time are the sameas those heretofore described in conjunction with the conditionsexisting at the time stations and 2 are communicating with each otherand station 3 selects station 2.

Assuming, now, that station 2 selects station 3 and not station I, thentubes |48 and |50 at stations 2 and 3 are extinguished while tubes |45and |41 at stations I and 2 glow continuously. The circuit conditionsexisting at this time are the same as those previously described inconjunction with the situation wherein stations I and 2 werecommunicating, station 3 called station 2 and station 2 gave precedenceto station 3 over station I.

A conference between all of the stations of the system may also bearranged. In this event, the station desiring to arrange the conferencecan call the other stations of the system and the operators at the otherstations can then operate all the station selector switches thereat toconnect the communication circuits for conference purposes.

Assuming that station I desires to arrange the conference, then theoperator at station I operates the station selector switches 83 and 84corresponding to stations 2 and 3. Tubes |45 and |48 at station i andalso tube |41 at station 2 and tube |49 at station 3 are thereuponsupplied with periodic voltage in a manner heretofore described, and theenumerated tubes thus flash on and off. The operators at stations 2 and3 then first operate selector switches 99 and I|2 conditioning therespective stations for communication with station I. The operator atstation then operates the communication control switch I1 thereat to itstalk position and informs the operators at stations 2 and 3 of thedesire to arrange a conference. The operators at the called stationsthereupon operate selector switches |00 and II3, respectively, with thenet result that all the station selector switches in the system areclosed. At this time, the tubes |45 to |50 are all extinguished inasmuchas the tubes are all disconnected from the common conductor |52. Afterthe circuits have been arranged for the conference, the stations canindividually transmit to the other stations by operation of thecommunication control switches thereat to their talk positions, thecommunication control switches at the other stations being in theirlisten positions.

In a system comprising more than three stations, if less than all of the'stations are arranged for a conference, any other station selecting oneof the conferring stations will be given a signal that the calledstation is busy in the same manner that the third station of thespecific embodiment illustrated is given a signal upon the selection ofa busy station.

In Fig. 5 there is illustrated a modified circuit arrangement forenabling one station to signal another of a desire to break in upon thetransmission from the other station to said one station. The circuitarrangement of this figure corresponds closely to that previouslydescribed except for the connections of the break-in signal tubes I I6and I I1 at stations I and 2, and like elements in this gure have beenindicated by the reference characters previously used. The tube ||8 atstation I is connected directly to the B supply conductor 38 throughresistor ||9 and to the midpoint of the output transformer secondarywinding 4I of amplifier I4 by a conductor 240. The tube I I1 at station2 is similarly connected to the B supply conductor 38 at that stationthrough resistor I 24 and to the midpoint of the secondary winding ofthe output transformer of ampliler I5 by conductor 24|. The energizingcircuit for tube II1 at station 2 is completed at station I through aconductor 242 leading from the midpoint oi' the primary winding 15 oftransformer 15 to a contact associated with a switch blade 243 formingpart of and operable with the remaining switch blades of thecommunication control switch I1. The switch blade is grounded by aconductor 244'. The energizing circuit for tube ||5 at station I iscompleted through a similar circuit arrangement at station 2 including a'conductor 245 leading from the midpoint of the primary winding oftransformer 84 to a contact associated with switch blade 245 formingpart of and operable simultaneously with the remaining switch blades ofcommunication control switch I5. The switch blade is grounded byconductor 241.

The operation o1' the arrangement illustrated in Fig. 5 is the same asthat of the previous embodiment. When the communication control switchesat the two stations are in their uppermost or listen positions. thetubes |I5 and II1 are both deenergized. When the two -stations have beenplaced in communicating relationship with each other by the closure ofthe station selector switches 83 and 58, then the tubes remaindeenergized when the communication control switch at one station is inits listen position and the communication control switch at the other isin its talk position. When both switches are in their talk positions, asis the case when a station conditioned for reception desires tointerrupt a station conditioned for transmission, then the energizingcircuits for the two tubes are completed simultaneously. The energizingcircuit for tube I|5 at station I is completed across the B supply atthat station. the circuit extending from the B supply conductorV 38through resistor IIS to the tube II5 and thence to ground throughconductor 240, the midpoint of secondary winding 4I of amplier I4, theoutput conductors 42, switch blades 8| of switch I1, the output circuit52, switch blades 85 in their closed positions, interstation circuit 85,the midpoint of the primary winding of transformer 84 at station 2,conductor 245, switch blade 245 in its lowermost position, and theground connection 241. The energizing circuit for tube II1 at station- 2is similarly completed from the B supply conductor 35 at that station toground as follows: conductor |24, tube II1, conductor 24|, midpoint ofthe secondary winding of the output transformer of amplitler I5, outputconductors 86, switch blades 85, the output circuit 85, switch bladesIIII, interstation circuit 13, midpoint of primary winding at station I,conductor 242, switch blade 243 in its lowermost position, and theground connection 244.

The modification of the invention illustrated in Fig. 6 differs somewhatfrom those previously described in that the neon tube at a stationconditioned for reception from another station conditioned fortransmission is energized while the tube at the station conditioned fortransmission is not energized until the station conditioned forreception operates the Vcommunication control switch thereat tocondition it for transmission.

Referring now to Fig. 6, in which elements corresponding to thosepreviously described are indicated by like reference characters, it maybe noted that the tube I|5 at station I is connected to the midpoint ofthe primary winding of the transformer 15 through conductor 25| andresistor 252 and to ground by a conductor 253. In like manner, the tubeII1 at station 2 is connected to the midpoint of the primary winding oftransformer 84 by conductor 254 and resistor 255 and to ground by aconductor 255. .The e'nergizing circuit for tube ||5 at station I iscompleted through a circuit at station 2 including a conductor 251leading from the midpoint oi' secondary winding of the outputtransformer of ampliiier I5 to a switch blade 255 through a resistor255. Ihe switch blade 255 forms part of the communication control switchI5 and is adapted when the latter is in its talk position to complete acircuit to the B supply conductor 35 through a branch conductor 258.'I'he energizing circuit for tube II1 at station 2 is similarlycompleted at station I through a conductor 25| leading from the midpointof the secondary winding 4| oi' ampliiier I5 to switch blade 252 througha resistor 255. The switch blade 252 forms part of the communicationcontrol switch I1 and is adapted to complete a circuit to the B supplyline 35 when the switch I1 is in its talk position through a branchconductor 254.

After stations I and 2 have been placed in communication with each otherby the closure of the station selector switches 53 and l88, then withboth the communication control switches I1 and I5 in their listenpositions, both tubes II5 and II1 are deenergized. When the switch I1 atstation I is in its talk position and switch I8 at station 2 is in itslisten position, tube II1 at station 2 is energized through a circuitincluding the interstation conductors 84 individual' to station 2 andthe switch blade 252 in its circuit closing position wherein it connectsthe B supply line 35 to the interstation circuit. When the switch I 1 isin its listen position and switch I8 in its talk position. tube II6 atstation I is. energized through a similar circuit including theinterstation circuit 13 individual to station I and switch lblade 255 incircuit closing position.

When one of the stations desires to interrupt the other stationconditioned for transmission, it is necessary only to operate thecommunication control switch thereat toits talk position. When this isdone, the neon tube at the station originally conditioned fortransmission is energized to give an indication to the operator thereatthat the other station desires to interrupt or break in on thetransmission.

It may be well again to note that While only a comparatively fewstations have been illustrated in the description of the variousembodiments of the invention, it should be understood that the number ofstations may be increased as required by the needs of the particularinstallation. While the invention has been disclosed in conjunction withthe specic details of preferred embodiments thereof, it should beunderstood that such details are not intended to be limitative of theinvention except in so far as set forth in the accompanying claims.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent of the United States is:

1. In an intercommunicating system of the type comprising a stationhaving a communication direction control switch operable to conditionthe station for reception or transmission and a second station, thecombination including, break in signal means at the first station, andmeans including signal means control circuits and a, switch at thesecond station controlling said circuits so constructed and arrangedthat the signal means at the iirst station can be operated from thesecond station when the first is conditioned for transmission and thesecond for reception for providing a signal at the station conditionedfor transmission that it is desired to break in on the transmission fromthe station.

2. In an intercommunicatng system oi the type comprising a first stationhaving a communication direction control switch operable to conditionthe station for reception or transmission and a second station, thecombination including, break in signal means at the rst station, andmeans including a switch at the second station i'or operating saidsignal means when the communication direction control switch at thei'lrst station conditions that station for transmission, thereby toindicate a desire to break in on the transmission from the firststation.

3. In an intercommnicating system of the type comprising a ilrststationhaving a communication direction control switch operable to conditionthe station for reception or transmission, a second station and acommunication circuit between the stations, the combination including,signal means at the nrst station, and means including said communicationcircuit and a switch at the second station for operating said signalmeans when the communication direction control switch at the firststation conditions that station for transmission, thereby to indicate adesire to break in on the transmission from the iirst station.

4. In an intercommunicatingsystem oi the type including a. rst stationhaving a communication direction control switch operable to conditionthe station for reception or transmission and a second station, thecombination including, signal means at said iirst station, signalcontrolling means at said iirst station for conditioning said signalmeans for operation when said communication direction control switchconditions said station ior transmission, and means including signal`controlling means at the second station for operating said signal meanswhen said first station is conditioned for transmission. A

'including a first station having a communication direction controlswitch operable to condition the station for reception or transmissionand a second station, the combination including, signal means at saidfirst station, switch means at said iirst station' operablesimultaneously with said communication direction control switch forconditioning said signal means for operation when said communicationdirection control switch is operated to condition said station fortransmission, and means including switch means at the second station foroperating said signal means when said iirst station is conditioned fortransmission.

6. In an intercommunicating system oi' the type including a firststation having a communication direction control switch operable tocondition the station for reception or transmission and a secondstation, the combination including, a signal light at said rst station,a source of power for said light, switch means at said rst stationoperable simultaneously with said communicationl direction controlswitch for partlycompleting the connection of said light to said sourceof power when said communication direction control switch is operated tocondition said station for transmission, and means including switchmeans at the second station for completing the connection of said lightto' said source of power when said first station is conditioned fortransmission.

7. In an intercommunicatng system oi the type Comprising a Pair fstations each having a communication direction control switch operableto condition the station for reception or transmission, the combinationincluding, break in signal means at each station, and means including .aswitch at each station for operating the signal means at another whenthe communication direction control switch at said other stationconditions said other station tor transmission, thereby to indicate avdesire to break in on the transmission from said other station.

8. In an intercommunicatng system of the type comprising a pair ofstations -each having a communication direction control switch operableto condition the station for reception and transmission, the combinationincluding, signal means at each station, signal controlling means ateach station for conditioning the signal means at that station i'oroperation when the communication control switch thereat conditions thestation for transmission, and signal controlling means at each stationoperable when the station is conditioned for transmission for operatingthe signal means at the other station when the signal means at the otherstation is conditioned for operation.

9. In an intercommunicatng system oi the type comprising a pair oi'stations each having a communication direction control switch operableto condition the station for reception and transmission, the combinationincluding, signal means at each station, signal controlling means ateach station operable simultaneously with the communication directioncontrol switch thereat for conditioning the signal means at that stationior operation when the communication control switch thereat conditionsthe station for transmission, and signal controlling means at eachstation operable simultaneously with the communication direction controlswitch thereat and operable when the station is conditioned fortransmission for operating the signal means at the other station whenthe signal means at the other station is conditioned for operation.

l0. In an intercommunicatng system of the type comprising a pair ofinterconnected stations having communication direction control switchesalternately operable to condition the stations for reception andtransmission, the combination including, signal means at each station,and means including signal means control circuits and switch meansoperable simultaneously with the ,communication direction controlswitches controlling said circuits so constructed and arranged that thesignal means at a station conditioned for transmission can be operatedfrom a station conditioned for reception upon operation of thecommunication direction control switch at the latter station tocondition it for transmission for providing a signal that it is desiredto break in on the transmission from the station.

11. In an intercommunicatng system of the type comprising a pair ofstations having com munication direction control switches alternatelyoperable between talk and listen positions for providing two waycommunication between the stations, the combination including signalmeans at each station, and signal controlling means at each stationoperable simultaneously with the communication control switch thereat soconstructed and arranged that the signal means at a stationconditionedfor transmission is operated when the communication directioncontrol switch at the other station is operated to 76 tionithe otherstation for transmission.

condi- 12. In an intercommunicating system of the type comprising a pairof stations interconnected by a pair of interstation circuits and havingcommunication direction control switches alternately operable betweentalk and listen positions for providing alternate two way communicationbetween the stations in opposite directions over said interstationcircuits, the combination including. signal means at one stationassociated with one of said circuits, signal means at the other stationassociated with the other of said circuits, and signal controlling meansat each station operable simultaneously with the communication controlswitch thereat so constructed and arranged that thesignal means at astation conditioned for transmission is operated when the communicationdirection control switch at the other station is operate-d to conditionthe other station for transmission. i

13. In an intercommunicating system of the type comprising a pair ofstations having communication direction control switches alternatelyoperable between talk and listen positions for providing two waycommunication between the stations, the combination including, a signallight at each station, a source of power at each station, and signalcontrolling means at each station operable simultaneously with thecommunication control switch thereat so constructed and arranged thatthe light at a station conditioned for transmission is connected to asource of power and energized when the communication direction controlswitch at the other station is operated to condition the other stationfor transmission.

14, In'an intercommunicating system of the type comprising a pair ofstations interconnected by a communication circuit and havingcommunication direction control switches alternately operable betweentalk and listen positions for providing two way communication betweenthe stations, the combination including, signal means at each stationconnected to said communication circuit in balanced manner, and signalcontrolling means at each station connected in balanced manner to saidcommunication circuit and operable simultaneously with the communicationcontrol switch thereat so constructed and arranged that the signal meansat a station conditioned for transmission is operated through saidcommunication circuit when the communication direction control switch atthe other station is operated to condition the other station fortransmission.

15. In an intercommunicating system of the type comprising a pair ofstations interconnected by a pair of interstation circuits and havingcommunication direction control switches alternately operable betweentalk and listen positions for providing alternate two way com--munication between the stations in opposite directions over saidinterstation circuits, the combination including, a signal light at eachstation connected in balanced manner to one of said circuits, a sourceof power at each station, and signal controlling means at each. stationoperable simultaneously with the communication control switch thereatfor conditioning the signal light at the station for connection to asource of power when the communication direction con, trol switchconditions the station for transmission, and means including signalcontrolling means at each 'station connectedl in balanced manner to acommunication circuit and operable simultaneously with the communicationdirection control switch thereat for completing the connection of thelight at the other station to a source of power.

16. In an intercommunicating system of the type comprising two or morestations adapted to be connected by a pair of interstation circuits anda common conductor and each having an ampliiier, a loudspeakerutilizable both as a receiver and a transmitter, station selector-means, and a communication direction control switch operable tocondition the station for reception over one of said circuits byconnection of the loudspeaker thereto or for transmission over the otherby connection oi' the loudspeaker to the input of the amplifier and ofthe output thereto through the station selector means, whereby two waycommunication can be carried on between two stations by alternateoperation of the switches thereat, including in combination, atransformer in the reception circuit at each station having a primarywinding connected to said one circuit thereat and a secondary windingadapted to be connected to the loudspeaker through said communicationdirection control switch when the latter conditions the station forreception, a signal light ateach station having one terminal connectedto the mid point of the primary winding of the transformer at thatstation, a source of power at each station, switch ymeans operable withthe communication direction control switch at each station for partlycompleting the connection of the light at the station to the source ofpower thereat by connecting the other terminal of the light to thesource when the communication direction control switch conditions thestation for transmission, a pair of resistors connected across theamplifier output at each station, a connection from between theresistors i-o said common conductor, and other switch means forming partof and operable with the communication direction control switch at eachstation for completing the connection of the light at the other stationto the source of power at the other station through said commonconductor and transmission circuit when the communication directioncontrol switch conditions the station for transmission, whereby thesignals at both stations are connected to their respective sources ofpower when both stations are conditioned for transmission.

17. In an intercommunicating system of the type comprising two or morestations adapted to be connected by a pair of interstation circuits andeach having a communication direction control switch operable tocondition the station for reception over one of said circuits or fortransmission over the other, whereby two way communication can becarried on between two stations by alternate operation of the switchesthereat, including in combination, a signal light at each stationconnected to the reception circuit for that station. a source of powerat each station, switch means operable with the communication directioncontrol switch atveach station for partly completing the connection ofthe light at the station to the source of power thereat when thecommunication direction control switch conditions the station fortransmission, and other switch means operable with the communicationdirection control switch at each station for completing the connectionof the light at the other station to the source of power at the otherstation through said transmission circuit when the communicationdirection contml switch conditions the station for transmission, wherebythe signals at both stations are connected to their respective sourcesof power when both' stations are conditioned for transmission.

18. In an intercommunicating system of the type comprising two or morestations adapted to be connected by a pair of interstation circuits anda common conductor and each having a communication direction controlswitch operable to condition the station for rec ption over one of saidcircuits or for transm ssion over the other, whereby two waycommunication can be carried on between two stations by alternateoperation of the switches thereat, including in combination, atransformer in the reception circuit at each station, a signal light ateachi station connected to the mid point of the primary winding of thetransformer at that station, a source of power at each station, switchmeans operable with the communication direction control switch at eachstation for partly completing the connection of the light at the stationto the source of power thereat when the communication direction controlswitch conditions the station for transmission, a pair of resistorsconnected across the transmission circuit at each station, a connectionfrom between the resistors to said common conductor, and other switchmeans forming part of and operable with the communication directioncontrol switch at each station for completing the connection of thelight at the other station to the source of power at the other stationthrough said common conductor when the communication direction controlswitch conditions the station for transmission, whereby the lights atboth stations are connected to their respective sources of power whenboth stations are conditioned Afor transmission.

19..In an intercommunicating system of th type comprising two or morestations adapted to be connected by a pair of interstation circuits andeach having a communication direction control switch operable tocondition the station for reception over one of said circuits fortransmission over the other, 'whereby two way communication can becarried on between two stations by alternate operation of the switchesthereat, including in combination, a source of power at each station, asignal light at each station having one terminal connected to the sourceof power at that station, switch means operable with the communicationdirection control switch at eachvstation for connecting the otherterminal of the light at the station to said other circuit when thecommunication direction control switch-conditions the station fortransmission over said other circuit, and other switch means operablewith the communication direction control switch at each station forcompleting the connection of the light at the other station to thesource of power at the other station through said one circuit when thecommunication direction control switch conditions the station fortransmission, whereby the signals at both stations are connected attheir source of power when both stations are conditioned fortransmission.

20. In an intercommunicating system of the type comprising two or morestations adapted to be connected by a pair of interstation circuits anda common conductor and each having a communication direction controlswitch operable to condition the station for reception over one of saidcircuits or for transmission over the other. whereby two waycommunication can be carried on between two stations by alternateoperation of the switches thereat, including in combination, a sourceofl power at each station, a transformer in the reception circuit ateach station, a connection from the mid point of the primary winding ofthe transformer at each station to the communication direction controlswitch,lswitch means operable with the communication direction controlswitch at each station for connecting the common conductor to saidconnection when the communication direction control switch conditionsthe station for transmission, a signal light at each station having oneterminal connected to the source of power thereat, and other switchmeans forming part of and operable with the communication directioncontrol switch at each station for connecting the other terminal of thelight at the station to the transmission circuit thereat and therebycompleting the connection of the light at the other station to thesource of power at the other station when the communication directioncontrol switch conditions the station for transmission, whereby thelights at both stations are connected to their respective sources ofpower when both stations are conditioned for transmission.

21. In an intercommunicating system of the v type comprising two or morestations adapted to be connected by a pair of interstation circuits anda common conductor and each having an amplifier, a loudspeakerutilizable both as a receiver and a transmitter, station selector means,and a communication direction control switch operable to condition thestation for reception over one of said circuits by connection of theloudspeaker thereto or for transmission over` the other by connection ofthe loudspeaker to the input of the amplifier and of the output theretothrough the station selector means, whereby two way communication can becarried on between two stations by alternate operation of the switchesthereat, including in combination, a transformer in the receptioncircuit at each station having a primary winding connected to said onecircuit thereat and a secondary winding adapted to be connected to theloudspeaker through said communication direction control switch when thelatter conditions the station for reception, a connection at eachstation from the mid point of the primary winding of the transformer atthat station to the communication direction control switch, switch meansoperable with the communication direction control switch at each stationfor connecting the connection at the station to the common conductorwhen thevcommunication direction control switch conditions the stationfor transmission, a signal light at each station having one terminalconnected to the source of power and its other terminal to the amplieroutput circuit, and other switch means forming part of and operable withthe communication ldirection control switch at each station forcompleting the connection of the light at the other station to thesource of power at the other station through said common conductor andtransmission circuit when the communication direction control switchconditions the station for transmission, whereby the signals at bothstations are connected to their respective sources of power when bothstations are conditioned for transmission. y

22. In an intercommunicating system of the type comprising two or morestations adapted to be connected by a pair oi interstation circuits anda common conductor and each having a communication direction controlswitch operable to condition the station for reception over one o1'.

said circuits for transmission over the other, whereby two waycommunication can be carried on between two stations by alternateoperation of the vswitches thereat, including in combination, a signallight at each station connected to the reception circuit for thatstation and to the common conductor, a source of power at each station,switch means operable with the communication direction control switch ateach station for connecting the light at the other station to the sourceof power at the station through the transmission circuit and commonconductor when the communication direction control switch conditions thestation for transmission, whereby the signal light at a stationconditioned for transmission may be connected to a source of power uponoperation oi the communication direction control switch at a stationconditioned for reception to condition the latter station for reception.

23. In an intercommunicating systemoi' the type comprising two or morestations adapted to be connected by a pair of interstation circuits anda common conductor and each having a communication direction controlswitch operable to condition the station for reception over one of saidcircuits or for transmission over the other, whereby two waycommunication can be carried on between two stations by alternateoperation oi' the switches thereat, including in combination, atransformer in the reception circuit at each station, a signal light ateach station connected to the mid point of the primary winding of thetransformer at that station and to the common conductor, a source ofpower at each station, and switch means forming part of and operablewith the communication direction control switch at each station forconnecting the light at the other station to the source oi power at thestation through said transmission circuit common conductor when thecommunication direction control switch conditions the station fortransmission, whereby the signal light at a station conditioned forreception is connected to the source of power at a station conditionedfor transmission and a station conditioned'for reception from oneconditioned for transmission can signal the latter by conditioning thestation for transmission.

24. In an intercommunicating system of the type comprising two or morestations adapted to be connected by a pair oi interstation circuits anda common conductor and each having an amplifier, a loudspeakerutilizable both as a receiver and a transmitter, station selector means,and a communication direction control'switch operable to condition thestation for reception over one of said circuits by connection of theloudspeaker thereto or for transmission over the input of the amplierand of the output thereto through the station selector means, wherebytwo way communication can be carried on between two stations byalternate operation of the switches thereat, including in combination, atransformer in the reception circuit at each station having other byconnection of the loudspeaker to the signal light at each stationconnected across the mid point of the primary winding of the transformerat that station and the common con-` ductor, a source of power at eachstation, switch means operable with the communication direction controlswitch at each station for connecting the source of power thereat to thetransmission circuit when the communication direction control switchconditions the station for transmission, whereby the signal light at astation conditioned for reception is connected to the source of power ata station conditioned for transmission and a station conditioned forreception from one conditioned for transmission can signal the latter byconditioning the station for transmission.

25. In a multiple station intercommunicating .system of the loudspeakertype comprisinga plurality of stations. any two of which can be placedin communication with each other over two interstation circuits, each ofwhich is utilized for communication in one direction only, and eachstation having a communication direction control switch operable tocondition the station tor reception or transmission, the combinationincluding, signal means at each station, and

means including switch means operable simultaneously with saidcommunication direction control switches and circuits controlled therebyfor operating the signal means at a station conditioned for transmissionfrom a station conditioned for reception upon operation of thecomoperable simultaneously with said communication direction controlswitches and circuits controlled thereby for operating the signal meansat a station conditioned for transmission `from a station conditionedfor reception upon operation of the communication direction controlswitch at the station conditioned for reception to condition thatstation for transmission.

27. In a multiple station intercommunicating system.' the combinationincluding, signal means at each station, station selector switches ateach station, means including circuits between stations controlling thesignal means and controlled by the station selector switches foreiecting operation of said signal means to produce dilerent distinctivesignals at both selected and selecting stations when a station selectsanother dependent trolling the signal means and controlled by said aprimary winding connected to said one circuit switch means for eiectingoperation of said signal means to produce diierent distinctive signalsat the station at which switch means is operated -and the stationcorresponding to the operated switch means dependent upon whether or notswitch means has been operated at the latter station.

29. In a multiple station intercommunicating system, the combinationincluding, signal lights at each station corresponding to other stationsin the system, switch means at each station corresponding to otherstations of the system, means including circuits between stationscontrolling the signal lights and controlled by said switch means forselectively eiecting steady or periodic energization of said signallights to produce different distinctive signals at thestation at whichswitch means is operated and the station corresponding to the operatedswitch means dependent upon whether or not switch means has beenoperated at the latter station.

30. In a multiple station intercommunicating system, the combinationincluding, signal lights at each station corresponding to other stationsin the system, station selector switches at each station correspondingto other stations of the at each station corresponding to other stationsin the system, station selector switches at each station correspondingto other stations of the system, a source of steady voltage. a source oiperiodic voltage, means including circuits between stations controllingthe signal lights and light circuit controlling switches operable withsaid selector switches for selectively connecting said signal lights tosaid periodic or steady voltage sources to produce different distinctivesignals at the station at which a selector switch is operated and thestation corresponding tothe operated selector switch dependent uponwhether or not a selector switch has been operated at the latterstation.

32. In a multiple station intercommunicating system, the combinationincluding, neon tubes at each station corresponding to other stations ofthe system, station selector switches at each station corresponding toother stations of the system, a source of steady voltage, a rstconductor leading therefrom to all stations, a source of periodicvoltage, a second conductor leading therefromto all stations, a commonconductor connected Ato both voltage sources and leading to allstations, and means operable conjolntly with said selector switches forproducing a iiashing of tubes at both a selecting and selected stationif no selector switches are operated at the selected station and asteady glow of the tubes in the event a selector switch has beenoperated at the selected station, said means including conductorsconnecting in series the tubes at each pair of stations corresponding tosaid stations, a pair of switches operable with each selector switch,the first of said switches being connected to a terminal of itsassociated tube and operable when the selector switch is not operated toconnect the tube to the common conductor and when the selector switch isoperated to connect the tube to a conductor leading to a second of saidpair of switches asso--V ciated with the selector switches at otherstations corresponding thereto, the second of said pair of switches ateach station being connected in series to said last mentioned conductorand operable when the selector switch is not operated to connect saidlast mentioned conductor to said second conductor through othercorresponding switches at the station and when the selector switch isoperated to connect said last mentioned conductor to said ilrstconductor.

33. In a multiple station intercommunicating system, the combinationincluding, signal means at each station, station selector switch meansat each station individual to other stations of the system, meansincluding circuits between stations controlling the signal means andcontrolled by said switch means for effecting operation of said signalmeans to produce a periodic signal or another signal at the station atwhich switch means is operated dependent upon whether or not switchmeans has been operated at the station corresponding to the operatedswitch means.

34. In a multiple station intercommunicating system, the combinationincluding, neon tubes at each station corresponding to other stations ofthe system, station selector switches at each station corresponding toother stations of the system, a source of steady voltage, a iirstconductor leading therefrom to all stations, a source of periodicvoltage, a second conductor leading therefrom to all stations, a commqiconductor connected to both voltage sources and leading to all stations,and means operable conjointly with said selector switches for producinga flashing of a tube at a selecting station if no selector switches areoperated at the selected station and a steady glow of the tube in theevent a selector switch has been operated at the selected station, saidmeans including conductors connecting the tubes at each station to thestations corresponding thereto, a pair of switches operable with eachselector switch, a first of said switches being connected to a terminalof its associated tube and operable when the selector switch is notoperated to connect the tube to the common conductor and when theselector switch is operated to connect the tube to a conductor leadingto a second of said pair of switches associated with the selectorswitches at other stations corresponding thereto, the second of saidpair of switches at each station being connected in series to said lastmentioned conductor and operable when the selector switch is notoperated to connect said last mentioned conductor to said secondconductor through other corresponding' switches at the station and whenthe selector switch is operated to connect said last mentioned conductorto said iirst conductor.

35. In a multiple station intercommunicating system, the combinationincluding, signal means at each station, switch means at each stationcorresponding to other stations of the system, means including circuitsbetween stations controlling the signal means and controlled by saidswitch means for eifecting operation of said signal means to produce aperiodic signal or another signal at the station corresponding to theoperated switch means dependent upon whether or not switch means hasbeen operated at the latter station.

36. In a multiple station intercommunicating system, a source oi steadyvoltage, a ilrst conductor leading therefrom to all stations, a sourceof periodic voltage, a second conductor leading therefrom to allstations, a common conductor connected' to both voltage sources andleading to all stations, and means operable conjointly with saidselector switches for producing a flashing of a tube at a selectedstation if no selector switches are operated at the selected station anda steady glow of the tube in the event a selector switch has beenoperated at the selected station, said means including a conductorconnecting the tubes at each station to corresponding stations, a pairof switches operable with each selector switch, a rst of said switchesbeing connected to a terminal of its associated tube and operable whenthe selector switch is not operated to connect the tube to the commonconductor and when the selector'switch is operated to connect the tubeto a conductor leading to a second of said pair oi switches associatedwith the selector switches at other stations corresponding thereto, thesecond of said pair-of switches at each station being connected inseries to said last mentioned conductor and operable when the selectorswitch is not operated to connect said last mentioned conductor to saidsecond conductor through other corresponding switches at the station andwhen the selector switch is operated to connect said last mentionedconductor to said first conductor.

37. In a multiple station intercommunicatlngI system, the combinationincluding, signal means returning said signals to non-operated conditionupon operation of the switch means corresponding to the rst station atsaid corresponding station.

38. In a multiple station intercommunicatlng system, the combinationincluding, signal lights at each station corresponding to other stationsin the system, station selector switches at-each station correspondingto other stations of the system, a source of periodic voltage, meansincluding circuits between stations controlling the signal lights andlight circuit controlling switches operable with said selector switchesfor selectively connecting said signal lights to said periodic voltagesource to produce distinctive signals at the station at which a selectorswitch is operated and the station corresponding to the operatedselector switch at which a selector switch has not been operated and fordisconnecting said lights from said source of periodic voltage uponoperation of the selector switch corresponding to said first station atsaid corresponding station.

39. In a multiple station intercommunicatlng system, the combinationincluding, signalmeans at each station, switch means at each stationcorresponding to other stations ofthe system, means including circuitsbetween stations controlling the signal means and controlled by saidswitch means for effecting operation of said signal means to produce adistinctive signal at the station at which switch means is operated andthe station corresponding to the operated switch means and at whichswitch means has not been operated and for returning said signal meansto non-operated condition upon operation of the switch meanscorresponding to the first station at said corresponding station, andmeans including said circuits and switch means for effecting operationoi the signal means to produce other system, a source of steady voltage,a source of periodic voltage, means including circuits between stationscontrolling the signal lights and light circuit controlling switchesoperable with said selector switches for connecting said signal lightsto said periodic voltage source to produce distinctive signals at thestation at which a selector switch is operated and the stationcorresponding to the operated selector switch at which a selector switchhas not been operated and for disconnecting said signal lights from saidperiodicvoltage source upon operation of the selectorV switchcorresponding to said first station at said corresponding station, andmeans including said circuits and light circuit controlling switches forconnecting signal lights at a third station and one of said two firstmentioned stations to said source of steady voltage upon operation atsaid third station oi a selector switch corresponding to said onestation.

41. In a multiple station intercommunicating system, the combinationincluding, signal lights at each station corresponding to other stationsin the system, and means including switch means at each stationcorresponding to other stations in the system and controlling the signallights at-other stations for producing flashing of the signal lights atthe other station when switch means at said other stations occupypredetermined positions.v y

42. In a multiple station intercommunicating system, the combinationincluding, signal lights at each station corresponding to' otherstations in the system, and means including switch means .at eachstation corresponding to other stations in the system and controllingthe signal lights'at other stations for producing flashing of the signallights at the other stations in response to the operation of the switchmeans at the station in the event no switch means have been operated atsaid other stations.

43. In a multiple station intercommunicatlng system, the combinationincluding, signal lights at each station corresponding to other stationsin the system, and means including switch means at each stationcorresponding to other stations in the system and controllingthe signallights at other stations for producing flashing of the signal lights atthe other stations in response to the operation of the switch means atthe station in the event switch means have been operated at said otherstations.

44. In a multiple station intercommunicatlng system, the combinationincluding, signal lights at each station corresponding to other stationsin the system, and means including switch means at each stationcorresponding to other stations in the system and controlling the signallights for producing ilashingof the signal lights at each station inresponse to the operation of the switch means thereat when the switchmeans at stations corresponding to the operated switch means occupypredetermined positions.

45. In a multiple station intercommunicating system, the combi'iationincluding, signal lights at each station cori sponding to'other stationsin the system, and mans including switch means at each stationcorresponding to other stations in the system and controlling the signallights for producing flashing of` the signal lights at each station inresponse to the operation of the switch means thereat when the switchmeans at stations corresponding to the operated switch means have beenoperated.

46. In a multiple station intercommunicating system, the combinationincluding, signal lights at each station corresponding to other stationsin the system, and means including switch means at each stationcorresponding to other stations in the system and controlling the signallights for producing flashing of the signal ihts at each station inresponse to the operation of the switch means thereat when the switchmeans at stations corresponding to the operated switch means have notbeen operated.

47. In a multiple station intercommunicating system, the combinationincluding, signal lights' at each station corresponding to otherstations in the system, and means including switch means at each stationfor controlling signal lights at that and at other stations forproducing ashing of the signal lights at that and at other stationscorresponding to operated switch means when the switch means at theother stations corresponding to the operated switch means occupypredetermined positions.

48. In a multiple station intercommunicating system, the combinationincluding, signal lights at each station corresponding to the otherstations in the system, and means including switch means at each stationfor controlling signal lights at that and at other stations forproducing ilashing of the signal lights at that and at other stationscorresponding to operated switch means when the switch means at theother stations corresponding to the operated switch means have not beenoperated.

49. In a multiple station intercommunicating system, the combinationincluding, signal lights at each station corresponding to other stationsin the system, and means including switch means at each station forcontrolling signal lights at that and at other stations for producingashing 'of the signal lights at that and at other stations correspondingto operated switch means when the switch means at the other stationscorresponding to the operated switch means have been operated.

50. In a multiple station intercommunicating system, the combinationincluding, signal means at each station, station selector switches ateach station, means including circuits between stations controlling thesignal means and controlled by the station selector switches foreffecting operation of said signal means to produce differentdistinctive signals at both selected and selecting stations when astation selects another dependent upon whether or not a station selectorswitch has been operated at the selected station, and means forreversing the natures of the distinctive signals produced in response tothe selection of a station at which a selector switch has or has notbeen operated.

l. In a multiple `ttation intercommunicating system, the combinationincluding, signal lights at each station, station selector switches ateach station, means including circuits between stations controlling thesignal means and controlled by the station selector switches foreiecting operation of said signal means to produce steady glow orflashing of signa1 lights at both selected and selecting stations when astation selects another dependent upon whether or not a station selectorswitch has been operated at the selected station, and means forreversing the nature of the signals produced upon selection of a stationat which a selector switch has or has not been operated.

52. A periodic voltage supply system, including in combination, a sourceof voltage, a, voltage output conductor, switch means operablealternately to connect said conductor to and to disconnect said.conductor from said source, and means including a relay having awinding energized from said voltage source through said switch meanswhen the latter disconnects said output conductor'from said voltagesource for periodically operating said switch means.

53. A periodic voltage supply system, including in combination, a sourceof voltage, a voltage output conductor, switch means operable betweentwo positions alternately to connect said conductor to said source andto disconnect said conductor from said source, and means including arelay having a winding adapted to be energized from said voltage sourcethrough said switch means when the latter disconnects said outputconductor from said voltage source for periodically operating saidswitch means between its two positions.

54. A periodic voltage supply system, including in combination, e.source of voltage, a voltage output conductor, a relay having a Winding,a switch blade connected to said source oi? voltage and operable in oneposition to connect said source to said conductor and to disconnect saidrelay Winding from said source and in another position to disconnectsaid source from said conductor and to connect said source to said relaywinding, and means including a condenser associated with said relaywinding for determining the lengths oi time the switch blade is in itstwo positions.

55. A periodic voltage supply system, including in combination, a sourceof voltage, a voltage output conductor, a relay having a winding, aswitch blade connected to said source of voltagel operable in oneposition to connect said source to said conductor and disconnect saidrelay winding from said source and in another position to disconnectsaid source from said conductor and to connect said source to said relaywinding, and means including a condenser in circuit with said relaywinding and charged when said winding is connected to said source anddischarged when said winding is disconnected from the source fordetermining the lengths of time the switch blade is in its twopositions.

56. .A periodic voltage supply system, including in combination, asource of voltage, a voltage output conductor, a relay having a winding,a switch means operable in one position to connect said source to saidconductor and to disconnect said winding from said source and in anotherposition to disconnect said source from said conductor and to connectsaid source to said relay winding, and means including acondenser-resistor network in circuit with said relay winding andthrough which said relay winding is energized and discharging throughsaid winding 4when the winding is disconnected from said source fordetermining the

